A conventional manufacturing process for manufacturing a semiconductor element such as an LSI or VLSI formed from a micropattern uses a reduction type projection exposure apparatus for transferring by reduction projection a circuit pattern drawn on a mask such as a reticle onto a substrate coated with a photosensitive agent. With an increase in the packaging density of semiconductor elements, demands have arisen for further micropatterning. Exposure apparatuses are coping with micropatterning along with the development of a resist process.
Methods of increasing the resolving power of the exposure apparatus include a method of changing the exposure wavelength to a shorter one, and a method of increasing the numerical aperture (NA) of the projection optical system.
As for the exposure wavelength, a KrF excimer laser with an oscillation wavelength of 365-nm i-line to recently 248 nm, and an ArF excimer laser with an oscillation wavelength around 193 nm have been developed. A fluorine (F2) excimer laser with an oscillation wavelength around 157 nm is also under development.
An ArF excimer laser with a wavelength around ultraviolet rays, particularly, 193 nm, and a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm are known to have an oxygen (O2) absorption band around their wavelength band.
For example, a fluorine excimer laser has been applied to an exposure apparatus because of a short wavelength of 157 nm. The 157-nm wavelength falls within a wavelength region called a vacuum ultraviolet region. Light in this wavelength region is greatly absorbed by oxygen molecules. In other words, light hardly passes through the air. Thus, the fluorine excimer laser can only be applied in a low-oxygen-concentration environment. According to the reference “Photochemistry of Small Molecules” (Hideo Okabe, A Wiley-Interscience Publication, 1978, p. 178), the absorption coefficient of oxygen to 157-nm light is about 190 atm−1 cm−1. This means when 157-nm light passes through a gas at an oxygen concentration of 1% at one atmospheric pressure, the transmittance per cm is onlyT=exp(−190×1 cm×0.01 atm)=0.150.
In such an exposure apparatus using an ArF excimer laser with a wavelength around ultraviolet rays, particularly, 193 nm, or a fluorine (F2) excimer laser with a wavelength around 157 nm, an ArF excimer laser beam or fluorine (F2) excimer laser beam is readily absorbed by a substance. A light absorption substance in the optical path must be purged to several ppm order or less. This also applied to moisture, which must be removed to the ppm order or less.
To ensure the transmittance and stability of ultraviolet rays, the ultraviolet path of a reticle stage or the like in the exposure apparatus is purged with inert gas. For example, Japanese Patent Laid-Open No. 6-260385 discloses a method of spraying inert gas to a photosensitive substrate, which is not enough to purge oxygen or moisture. Japanese Patent Laid-Open No. 8-279458 discloses a method of covering the whole space from the lower end of a projection optical system to the vicinity of a photosensitive substrate with a sealing member. However, the stage is difficult to move, and this method is not practical. Japanese Patent Application No. 2000-179590 discloses a method of disposing a cover for covering the ultraviolet path from the reticle-side lower end of an illumination optical system to the vicinity of a reticle stage, and spraying inert gas into the cover. However, oxygen or moisture cannot be sufficiently purged because the space from a reticle holder for holding a reticle to a reticle surface plate for supporting a reticle stage is not surrounded. In addition, a sheet glass is set on the reticle stage side serving as a movable portion, which increases the weight of the reticle stage. Since the scan stroke range of the reticle stage must be covered with the sheet glass, a large sheet glass is required to further increase the weight. The sheet glass deforms by driving of the reticle stage, changing the optical characteristics. Particularly when the sheet glass functions as an optical element, changes in optical characteristics typically appear. In this case, the optical characteristics must be uniform within the scan stroke range, resulting in a very complicated process. Connection of an inert gas supply tube to the reticle stage side serving as a movable portion transmits vibrations from the tube. Scan operation while the tube is connected degrades the control characteristics of the reticle stage.
As described above, an exposure apparatus using an ultraviolet ray, particularly, an ArF excimer laser beam or fluorine (F2) excimer laser beam suffers from large absorption of light of this wavelength by oxygen and moisture. To obtain a sufficient transmittance and stability of an ultraviolet ray, the oxygen and moisture concentrations in the optical path must be reduced.
From this, demands have arisen for the development of an effective purge system for the ultraviolet light path in the exposure apparatus, particularly for the vicinity of a wafer and reticle which are often loaded/unloaded into/from the exposure apparatus.